A Light-Emitting Diode (LED), as used herein, is a semiconductor light source for generating a light at a specified wavelength or a range of wavelengths. LEDs are traditionally used for indicator lamps, and are increasingly used for displays. An LED emits light when a voltage is applied across a p-n junction formed by oppositely doping semiconductor compound layers. Different wavelengths of light can be generated using different materials by varying the bandgaps of the semiconductor layers and by fabricating an active layer within the p-n junction.
Traditionally, LEDs are made by growing a plurality of light-emitting structures on a growth substrate. The light-emitting structures along with the underlying growth substrate are separated into individual LED dies. At some point before or after the separation, electrodes or conductive pads are added to the each of the LED dies to allow the conduction of electricity across the structure. LED dies are then packaged by adding a package substrate, optional phosphor material, and optics such as lens and reflectors to become an optical emitter.
Optical emitter specifications typically identify application-specific radiation patterns outputted by the optical emitter. A commonly used beam pattern is a batwing beam pattern for illuminating a flat surface, in traffic signal applications, for a backlighting unit in a display, or for general lighting. The batwing beam pattern may be defined by having two roughly equal peaks in a candela distribution plot with a valley between the peaks at about 0 degrees. The batwing pattern may be defined by a uniformity, a viewing angle, a minimum output measured at zero degrees, full width at half maximum (FWHM), and peak angles. The uniformity defines the variability of the light output at different angles within certain angles of interest range, which may be the viewing angle. The viewing angle may be defined as the total angle at which 90% of the total luminous flux is captured. The minimum output at zero degrees is related to the uniformity. The peak angles determine the shape of the batwing and are related to the viewing angle. A full width at half maximum (FWHM) is the width (spatial or angular) of a distribution at half of its maximum value and is usually specified to be greater than about 100 degrees.
Optical emitters are designed to meet these specifications. While existing designs of optical emitters have been able to meet batwing beam pattern requirements, they have not been entirely satisfactory in every aspect. Smaller and more cost effective designs that are easier to manufacture continue to be sought.